The US National Aeronautics and Space Administration lost contact with a $158m (£105m) spacecraft early yesterday, when the robotic probe was to have left Earth orbit on a journey to explore several comets.

Nasa's Comet Nucleus Tour, or Contour, spacecraft was programed to fire its motor at 0849 GMT, boosting it from orbit.

At the time, the spacecraft was about 140 miles above the Indian Ocean.

Nasa's Deep Space Network of antennas was to have picked up a signal from Contour at 0935 GMT.

By late morning, the antennas in California, Australia and Spain still had not picked up a signal from the spacecraft.

"We really are still in a search mode, trying to communicate with the spacecraft," said Nasa spokesman Don Savage.

The mission's operations team was searching along the predicted paths that Contour might have taken in an effort to find it, Nasa said.

The spacecraft had been in Earth orbit since its launch on July 3. Yesterday's manoeuvre was to have sent it on its way toward encounters with at least two comets.

The mission plan called for it to meet up with comet Encke in 2003, Schwassman-Wachmann 3 in 2006, and perhaps comet d'Arrest in 2008.

The spacecraft was built by Johns Hopkins University, with assistance from Cornell University, for the National Aeronautics and Space Administration.